1. Field of the Invention
This invention relates to a Capacitance change detection circuit, a touch panel, and a determination method
2. Description of the Related Art
In recent years, touch panels used in various industrial equipment use touch sensors which detect which part of the touch panel a human has touched or contacted. When the human touches an electrode on the panel, the same state as a state in which a capacitor having electrostatic capacity equivalent to that of the human body is connected to the electrode is achieved. Utilizing this phenomenon, the touch sensor detects a change in electrostatic capacity formed between this electrode and ground (earth) as electric capacity, and determines whether or not there has been a touch or contact.
As such a touch sensor, one having a contact electrode exposed to the outside and an oscillator circuit connected to the contact electrode has been known (see Patent Document 1). With this touch sensor, when the human is out of contact with the contact electrode, the oscillator circuit performs steady oscillation. The touch sensor sets such an oscillation stop point that when the human contacts the contact electrode and the equivalent impedance of the human body exceeds a reference value specific to the circuit, the steady oscillation is stopped. The touch sensor also sets such an oscillation start point that when the human contacts the contact electrode and the equivalent impedance of an object to be detected exceeds a reference value specific to the circuit, oscillation is started. In this manner, the touch sensor outputs the states of contact and separation of the human body according to the presence and absence of steady oscillation.
As another touch sensor, one having a detection electrode in addition to the contact electrode and adapted to detect a capacitance difference between these electrodes has been known (see Patent Document 2). This touch sensor is equipped with a sensor case comprising an insulating material; a detection electrode disposed on the inner surface of the sensor case; a touch electrode disposed at a position on the outer surface of the sensor case, the position opposing the detection electrode, the touch electrode being capacitively coupled to the detection electrode; and a detection circuit connected to the detection electrode for detecting the contact of an object with the touch electrode based on a change in the electrostatic capacity between the detection electrode and ground and outputting a detection signal.
Patent Document 1: JP-A-2003-46383 (see claim 1 and FIG. 2)
Patent Document 2: JP-A-2004-340662 (see claim 1 and FIG. 1)